Each of your devices/apps has its own email address so you email directly to your iPad and iPhone, your Kindle email address is only for your Kindle. You can find the addresses associated with each device on your Manage Your Kindle page under Manage Your Devices. If you've emailed a document to one device, that document will be in your Amazon Library (cloud) and you can send it to your other devices from your Library instead of emailing it to each one if you find that easier.

Yes, I have tried that as well, but it seems that "grays" out "sync to furthest page read" on the K2, but the syncing works between the iphone, ipad and K4PC still.